This is a result of disc manufacturers and equipment manufacturers using different definitions of Kilobytes, Megabytes, and Gigabytes.
The decimal prefix definition uses kilo=1000, mega=1000*1000, and giga=1000*1000*1000
The binary prefix definition uses kilo=1024, mega=1024*1024, and giga=1024*1024*1024
Alternatively, the binary prefixes are named: Kibi, Mebi, and Gibi.
The 8.5 GB is the decimal prefix definition, meaning (at least) 8,500,000,000 bytes. This is equal to 7.916 Gibibytes (binary Gigabytes).
So the 0.6 Gigabytes isn't lost to formating or any such thing - they are "lost" due to different definitions being used.